At the end of Second World War, a number of different systems of measurement were in use throughout the world. Some of these systems were metric system variations, whereas others were based on customary systems of measure. It was recognised that additional steps were needed to promote a worldwide measurement system. After representations by the International Union of Pure and Applied Physics (IUPAP) and by the French Government, the 9th General Conference on Weights and Measures (CGPM), in 1948, asked the International Committee for Weights and Measures (CIPM) to conduct an international study of the measurement needs of the scientific, technical, and educational communities.[21]

Based on the findings of this study, the 10th CGPM in 1954 decided that an international system should be derived from six base units to provide for the measurement of temperature and optical radiation in addition to mechanical and electromagnetic quantities. The six base units that were recommended are the metre, kilogram, second, ampere, degree Kelvin (later renamed kelvin), and candela. In 1960, the 11th CGPM named the system the International System of Units, abbreviated SI from the French name, Le Système international d'unités.[22][23] The BIPM has also described SI as "the modern metric system".[24] The seventh base unit, the mole, was added in 1971 by the 14th CGPM.

Although, in theory, SI can be used for any physical measurement, it is recognized that some non-SI units still appear in the scientific, technical and commercial literature, and will continue to be used for many years to come. In addition, certain other units are so deeply embedded in the history and culture of the human race that they will continue to be used for the foreseeable future. The CIPM has catalogued such units and included them in the SI brochure so that they can be used consistently.

The first such group are the units of time and of angles and certain legacy non-SI metric units. Most of mankind has used the day and its subdivisions as a basis of time with the result that the second, minute, hour and day, unlike the foot or the pound, were the same regardless of where it was being measured. The second has been catalogued as an SI unit, its multiples as units of measure that may be used alongside the SI. The measurement of angles has likewise had a long history of consistent use - the radian, being 1⁄2π of a revolution has mathematical niceties, but is cumbersome for navigation, hence the retention of the degree, minute and second of arc. The tonne, litre and hectare were adopted by the CGPM in 1879 and have been retained as units that may be used alongside SI units, having been given unique symbols.

Physicist often use units of measure that are based on natural phenomena such as the speed of light, the mass of a proton (approximately one dalton), the charge of an electron and the like. These too have been catalogued in the SI brochure with consistent symbols, but with the caveat that their physical values need to be measured.[Note 7]

In the interests of standardising health-related units of measure used in the nuclear industry, the 12th CGPM (1964) accepted the continued use of the curie (symbol Ci) as a non-SI unit of activity for radionuclides;[68] the becquerel, sievert and gray were adopted in later years. Similarly, the millimetre of mercury (symbol mmHg) was retained for measuring blood pressure.

The system has been nearly globally adopted. Burma, Liberia and the United States have not adopted SI units as their official system of weights and measures. The U.S. does not commonly use metric units outside of science, medicine, and the government,[4] but has officially defined its customary units in terms of SI units. The United Kingdom has officially adopted a partial metrication policy, with no intention of replacing imperial units entirely. Canada has adopted it for most purposes but imperial units are still legally permitted and remain in common use throughout a few sectors of Canadian society, particularly in the buildings, trades and railways sectors.

The metric system was first implemented during the French Revolution (1790s) with just the metre and kilogram as standards. In the 1860s British scientists, working through the British Association for the Advancement of Science laid the foundations for a coherent system based on length, mass and time, but the inclusion of electrical units into the system was hampered until 1900 when Giorgi identified the need to define an electrical quantity alongside the original three quantities. Meanwhile, in 1875, the Treaty of the Metre passed custodianship of the prototype kilogram and metre from French to international control. In 1921 the Treaty was extended to include all physics measurements and in 1948 an overhaul of the metric system was set in motion which resulted in the publication of the International System of Units in 1960.

The metric system was developed from 1791 onwards by a group of scientists that was commissioned by the Assemblée nationale and Louis XVI of France to create a unified and rational system of measures.[9] The group, which included Antoine-Laurent Lavoisier (the "father of modern chemistry") and the mathematicians Pierre-Simon Laplace and Adrien-Marie Legendre,[10] used a number of principles first proposed by the English cleric John Wilkins in 1668[11] and the French cleric Gabriel Mouton in 1670.[12] On 1 August 1793, the National Convention adopted the new decimal metre with a provisional length as well as the other decimal units with preliminary definitions and terms. The law of 7 April 1795 (Loi du 18 germinal, an III) defined the terms gramme and kilogramme replaced the former terms gravet (correctly milligrave) and grave, and on 22 June 1799, after Pierre Méchain and Jean-Baptiste Delambre completed their survey, the definitive standard metre was deposited in the French National Archives. On 10 December 1799 (a month after Napoleon's coup d'état), the law by which metric system was to be definitively adopted in France was passed.

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In 1832 Carl Friedrich Gauss implicitly defined a coherent system of units when he measured the earth's magnetic field in absolute units quoted in terms of millimetres, grams, and seconds.[14] In the 1860s James Clerk Maxwell and William Thomson (later Lord Kelvin), working through the British Association for the Advancement of Science formulated the concept of a coherent system of units with base units and derived units. The principal of coherence was successfully used to define a number of units of measure based on the centimetre–gram–second (cgs) system of units (cgs) including the erg for energy, the dyne for force, the barye for pressure, dynamic viscosity in poise and the kinematic viscosity in stokes.